Cross-Site Request Forgery (CSRF) is a type of web security vulnerability that allows an attacker to execute unauthorized actions on behalf of a user who is authenticated. CSRF attacks are possible when a website uses session-based authentication, where the user's session is used to authenticate subsequent requests.

To prevent CSRF attacks, web developers need to implement defensive tactics such as:

1. Use CSRF Tokens: CSRF tokens are a unique token that is generated by the server and included in each form submitted by the user. The server then verifies the CSRF token to ensure that the request is legitimate.
2. Use Double Submit Cookie: The server can set a double-submit cookie that is sent to the user's browser. The cookie contains a unique token that is included in each form submitted by the user. The server then verifies the double-submit cookie to ensure that the request is legitimate.
3. Use Origin: The server can use the Origin header to verify that the request is coming from a trusted source. The Origin header contains the URL of the website that the request is being sent from.
4. Use Referer: The server can use the Referer header to verify that the request is coming from a trusted source. The Referer header contains the URL of the website that the user was visiting before clicking on the link that triggered the request.

By implementing these defensive tactics, web developers can significantly reduce the risk of CSRF attacks and ensure the security of their website and applications. It is important to note that CSRF attacks can be difficult to detect and prevent, so it is important to regularly monitor and update your website and application for security vulnerabilities.